The present invention relates to an underspeed actuator unit which is responsive to operating speed of a prime mover or engine in order to automatically adjust a variable drive coupling in a transmission driven by the prime mover. More particularly, the transmission is of a hydrostatic type including one or more pump units with the underspeed actuator being operable to adjust displacement of the pump or pumps in response to both speed variations of the transmission input and changes in output pressure from the pump unit or units.
Hydrostatic transmissions adapted for track-type vehicles commonly include parallel pump units each having variable displacement in order to permit operator control over vehicle speed and direction by appropriately varying displacement of one or both pumps. The present invention is particularly described below in connection with a hydrostatic transmission of this type. However, it will be apparent from the following description that the present invention is also applicable to more conventional hydrostatic transmissions having a single pump unit or multiple pump units whose displacement is always simultaneously varied as well as other types of transmissions.
In most hydrostatic transmissions, it is desirable to reduce the load placed upon the prime mover by the transmission when operating speed of the prime mover falls below a selected range because of an external load, such as when a vehicle including the prime mover and transmission is operating uphill or otherwise encounters an increased load.
The use of underspeed regulator or actuator valves is known in connection with hydrostatic transmissions for overcoming this problem as may be seen by reference to U.S. Pat. No. 3,477,225 and U.S. Pat. No. 3,727,628, both of which are assigned to the Assignee of the present invention. Another underspeed actuator is disclosed in a copending application entitled: UNDERSPEED ACTUATOR FOR HYDROSTATIC TRANSMISSION, filed on Feb. 20, 1975, as Ser. No. 551,474, assigned to the assignee of the present invention.
The underspeed actuators disclosed by these references are responsive only to a change in engine speed and do not begin to adjust pump displacement until engine speed is substantially varied. Normally, pressure changes which the pump encounters in the hydrostatic loop with an associated motor and resultant torque changes are relatively gradual allowing time for the underspeed actuator to suitably adjust displacement of the pump. However, when a load is suddenly imposed on the hydrostatic motor causing an immediate increase in the pump output pressure, there is a short time lag before engine speed starts to decrease and the underspeed actuator starts to react. Although the lag in time may generally be only momentary, the underspeed actuator tends to over-correct, thus developing a condition of "hunting" until pump displacement, pump output pressure and engine speed again stabilize. This time lag also permits detrimental pressure extremes in the hydrostatic loop between the pump and motor.